Learning Forum

[akban]

Hi,

I wish to simulate development of buoyant jet in a density stratified medium.

In my system, background density varies linearly from bottom (high) to top (low). Thereafter a jet of medium density is opened from bottom. So it should move up and at a neutral layer, the jet fluid should spread horizontally. Background fluid is water with varying density. At bottom density is 1016 kg/mt3 and at top density is 1001 kg/mt3 . Water of density 998 .3 kg/mt3 is impinged from bottom as round jet.

 I am adding two images to clarify my system.

1st image shows a tank full of water (density stratified as shown by different colours). Then a constant density water jet is impinged from bottom (shown by arrow). 2nd image shows experimental image that should be found in simulation.

[YasserSelima]

What is the boundary condition at the top?.

[akban]

No slip condition, assuming top boundary is far from neutral layer.n

[YasserSelima]

what happens if you add incompressible fluid in a filled room?n

[akban]

You mean my top boundary condition is wrong ? Should I use pressure outlet? or outflow?n

[Rob]

Pressure outlet is the better of the two options. You're adding mass into the domain so there has to be somewhere for it to go! n

[akban]

Even with pressure outlet on the top, the simulation is not providing desirable outcome.nnThe background stratified density do not mix with incoming jet density.nPlease suggest.n

[YasserSelima]

Can you show contours of Volume fraction of the impinged water n

[akban]

Please find contours of Volume fraction:nnThe jet fluid does not mix with background fluid.n

[YasserSelima]

It does mix. Keep the simulation running for some more time.nBTW, 202 seconds in two phase flow is a very large time. What is your time step?n

[akban]

My time step is 0.002.nnIn experiment, it creates a horizontal layer of fluid at neutrally buoyant height within 120 sec.n

[YasserSelima]

Plot void fraction of the dense water. (Not impinged) nIn the void fraction contours panel, uncheck the box auto range. Make the minimum 1e-06 and keep the maximum at 1. Then Select color map options and check the box log scale.nThis should give you a clearer view of how much the jet spread is.n

[akban]

What is this check the box log scale ?n

[YasserSelima]

After you select color-map option ... you will find a box Log scale ... check it and press ok ... then plot the contoursn

[akban]

n

[YasserSelima]

Make this contour phase with the same setting for phase 2. n

[akban]

n

[akban]

It should be like:nnn

[akban]

Moreover, density contour looks total junk.n

[YasserSelima]

Why is density contours changing? How did you setup the density at the beginning?.You created UDF, this is what I see at the top. Where did you hookup the UDF?n

[akban]

n

[YasserSelima]

yes, that is right. nAnd the density you showed at the top of this post was showing linear distribution. nCan you show contours of density of phase 1 at the current time. Phase 1, not mixturenn

[akban]

No change. Very strangennn

[YasserSelima]

It should not change ... the mixture density is different. Now before explaining what is in my mind, I need to see phase-2 contours, log scale .. minimum 1e-12 and ...nLast thing, Regarding the experiment ... how did you get the linear density? How did you capture this image? PIV? or what?.

[akban]

In experiment, linear density was created by using salt. The image is captured by a simple DSLR camera.n

[akban]

Phase 2 density looks junk.nBTW, what is in your mind?n

[YasserSelima]

The 2D simulation has an effect in obtaining this difference. Fluent recognises the jet depth is 1 m, which is not true ... To solve this issue, you need to have few cells in the z direction, , then apply symmetry plane. You can actually simulate 1 quarter of the room, and apply symmetry ... nMore smart solution ... make axisymmetry ... having rectangular or cylindrical room will not make a difference if it is large enough.nThe captured photo, the impinged fresh water particles, started to diffuse horizontally when the jet velocity became very small ... may be the salinity played a role.nFinally, I believe, if you left your simulation to continue, you might get something close to the photo .. nThe last density shows that the mixture density is similar to the impinged water density. I can't explain the physics of this ... it does not make sense . Probably this happened when you were running the simulation with wall at the top. !!nnmy recommendation, repeat the simulation ... asymmetric ... outlet pressure on top. And monitor the density in few points away from the jet.n

[akban]

I do have pressure outlet on the top (I changed B.C after recommendation). My simulation is already axisymmetric. Now what should I do? I can add elements in z direction.n

[YasserSelima]

axisymmetric? where is your axis?nIf the simulation is axisymmetric already, you need to introduce the salinity into the simulation. Have water+salt mixture (patch the initial concentration) ... and impinge water n

[YasserSelima]

In the axisymmetric, X must be the axis. n

[akban]

X is my vertical axis-- I have taken care of this.nWhy do I need to introduce the salinity into the simulation ? What is the problem if I have UDF of varying density?nHow do I make water+salt mixture ? Should I use species model ?.

[YasserSelima]

You need to introduce salinity because salinity difference causes diffusion. Nothing wrong with changing the density, but it did not replicate the experiment, so try to get closer to experiment.nYes, I would start by species model. Simple and it enables you to have mixture and pure water.

[akban]

Hi,nPlease suggest me how do I proceed to create salinity? nI chose species and mixing template. Then how should I choose materials to create salinity ?nPlease help.n

[YasserSelima]

yes switch on species transport, select volumetric and create a mixing template. Then press edit and you will be able to add material and specify density. You need to add the material in the material panel firs to find them when you select the mixture template n

[YasserSelima]

this tutorial is the only one I can find using species. It is not close to yours as you don't have combustion nor reaction, but go through it quickly.nhttps://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v202/en/flu_tg/flu_tg_magnus.htmln

[akban]

Hi,nI can not open the tutorial. BTW, in the edit of mixture template, how do I create stratified salinity? nn

[YasserSelima]

To open the tutorial, use the help button in fluent. This should open your browser. Copy the above link and paste it in the same page.nn